In a gas turbine power generating plant, air compressed by driving of a compressor provided coaxially with a turbine part is led to a combustor liner. A high-temperature and high-pressure combustion gas generated by the air led to the combustor liner and fuel being mixed to be burnt is led to the turbine part through a transition piece connected to the combustor liner. In the turbine part, the high-temperature and high-pressure combustion gas is expanded to thereby rotationally drive rotor blades and a turbine rotor, and by the rotational driving, the compressor to compress air and a power generator are driven.
FIG. 6 is a view showing a cross section of a conventional transition piece 200. The conventional transition piece 200, as shown in FIG. 6, has a double-shell structure composed of an inner duct 201 and an outer duct 202 provided around an outer periphery of the inner duct 201. One end of the inner duct 201 is coupled to a combustor liner 230 in a cylindrical shape, and the other end of the inner duct 201 is coupled to a stator blade 240 at a first stage of a turbine. Thus, the shape of a cross section, of a combustion gas channel 203 in the inner duct 201, perpendicular to a flowing direction of a combustion gas changes from a circular shape to a sector of annular shape. The outer duct 202 is also formed into a shape corresponding to the shape of the inner duct 201.
The inner duct 201 has the high-temperature combustion gas flow through the inside thereof, and thus is formed of a Ni-base superalloy, and further has a cooling structure. In the outer duct 202 in the transition piece of a typical gas turbine on order of 1300° C., a plurality of impingement cooling holes 204 through which part of air discharged from the compressor is ejected and made to impinge onto/on an outer surface of the inner duct 201 as cooling air 205 are formed over the entire surface as shown in FIG. 6.
At a downstream end of the transition piece 200, there is provided a flange-shaped picture frame 206 that seals one end between the inner duct 201 and the outer duct 202 to prevent outflow of the cooling air 205 to a stator blades 240 side.
The inner duct 201 of the above-described conventional transition piece 200 is formed of a Ni-base superalloy, and is cooled by the cooling air 205. However, when a base material increases in temperature locally while the gas turbine is in operation, damage such as cracks and thickness losses due to thermal fatigue and oxidation respectively is thereby caused in the inner duct 201.
In the conventional inner duct 201, deformation has been likely to occur in the vicinity of the picture frame 206. The above deformation tends to increase with an increase in operating time of the gas turbine, so that the deformation is conceivably caused by creep damage.
An outer surface side of the inner duct 201 receives pressure from the cooling air 205, and an inner surface side of the inner duct 201 receives pressure from the combustion gas. The pressure from the cooling air 205 is higher than that from the combustion gas, so that the inner duct 201 receives a load in a direction in which the inner duct 201 is pressed from the outside. Particularly, the cross-sectional shape of the inner duct 201 connected to the turbine part is not to be a cylindrical shape, so that the inner duct 201 connected to the turbine part is more likely to be deformed against the external pressure than the inner duct 201 connected to the combustor liner 230 having the cross-sectional shape being a circular shape. The external pressure to act on the above inner duct 201 also results in a cause of making the deformation occur easily in the vicinity of the picture frame 206.
Further, at a downstream side of the inner duct 201, the flow velocity of the combustion gas increases, so that a heat transfer coefficient with the combustion gas increases, the temperature of the inner duct 201 increases, and creep deformation is likely to occur. Further, by the combustion gas being increased in temperature and pressure with an increase in capacity of the gas turbine, the temperature of the inner duct 201 further increases, and the difference in pressure between a cooling air side and a combustion gas side of the inner duct 201 tends to increase. Thus, the condition that makes the creep deformation occur easily in the inner duct 201 is made.